text:
Is it possible to recover from a graveyard spiral?
The Graveyard spiral is an extremely dangerous situation and is the leading cause of a controlled flight into terrain accident. It is caused by the erroneous feedback from the vestibular system that the aircraft is in level flight when in fact it is descending and banking. However, my question is, is it possible to recover from a graveyard spiral1 in zero visibility conditions2? Even more importantly, in zero visibility conditions, will a pilots be able to even notice that the aircraft is in a graveyard spiral? If so, how do pilots notice the situation in zero visibility conditions, and what course of action should/will be taken to recover? I'll appreciate any insights on this, preferably from an experience pilot or flight instructor. Thanks in advance!

Notes:
1: It must have an extremely high fatality rate, it's called "graveyard" after all. The other names given by Wikipedia for this phenomenon seems even worse, "death spiral", "deadly spiral", "suicide spiral", "vicious spiral" all seem to convey the same meaning, eg. it's usually not survivable. Or are the names exaggerating things?
2: I know a common method to recover is to consciously override the vestibular  feedbacks and rely on visual cues instead. However, in zero visibility conditions, there's no outside visual cues, no horizon or the like. So it seems like it's going to be nearly impossible to recover if this is the case. Making life-or-death decisions relying only on electronic instruments that may be erroneous when your whole body is telling you otherwise is nearly impossible, I think. Or am I wrong? Do pilots tend to believe in their instruments more than their own body's senses?

If you're in zero visibility and you aren't looking at your artificial horizon, what are you even doing?

Any comment from downvoters? Please inform me of any possible improvements. I hope to make this post useful to future readers, so if there's anything I can improve, please tell me. I'm also welcome to any edits. I'll appreciate any help. Thanks in advance!

I don't specifically know about the downvotes, but I can say that some of your phrasing suggests that you aren't familiar with flight operations at all. That's not a sin in my view -- there's nothing wrong with ignorance, especially when the context is that you're actively trying to change that, and it's not like basic questions detract from the more complex/nuanced questions -- but this particular section of SE seems to be pretty hard on people who aren't deeply knowledgeable about aviation to start with.

Specifically, your footnote that "life-or-death decisions relying only on electronic instruments that may be erroneous" is quite the opposite of how pilots generally think. The instrument may fail, but if it fails it would (usually) be very obviously faulty, and airliners have redundant systems for anything critical. A mechanical system is much more trustworthy than your fallible wet meat that developed to operate in a generally two-dimensional environment at no more than a couple dozen miles per hour and is now being asked to comprehend a mid-air world moving at 10 to 20 times that speed.

Briefly, yes - you are wrong. It takes training, but it is not 'nearly impossible' by any stretch of the imagination. It is quite simple, truthfully, but it does take some training and practice. And once again, a proficient pilot will simply not get into such a situation, because he knows how to prevent it. It is not something that 'just happens' - it is the culmination of a series of mistakes, and takes a while to develop. A competent pilot will never let that happen.

Do pilots tend to believe in their instruments more than their own body's senses?

This is EXACTLY what you're taught to do in any instrument training,  and the reason that training is mandatory to fly in instrument meteorological conditions.  It is unnatural and does not come easily.
So while stall recovery is no doubt harder in IMC, and I would not like to try it,  the answer to your question is yes,  it is survivable as long as you trust your instruments and recover appropriately.

Doing stall and spin recoveries under the hood is an invaluable part of IFR training. Not many places even offer it, but you can always ask. And if they say no, it is worth doing some hunting to find a place that will.
Yes, of course, the better route is to not get into such a situation in the first place. But people make mistakes, and knowing how to recover from such a fantastically dangerous mistake can mean the difference between finishing your trip with soiled underwear and finishing it in a hearse.

Adding on to this: pilots are taught this not just because of a lack of visual references in many IMC scenarios, but because of the somatogravic illusion; the inner ear isn't good at differentiating acceleration from pitch, yaw, and roll moments along the same axes, which can lead to confusion. Working, calibrated instruments can effectively discriminate between all of that.

Voting down as slightly off topic.  The question never mentioned stalls or stall recovery.

@RobertChapin fair enough, but the question asks about a spiral, which is just another word for a spin and specifically says "So it seems like it's going to be nearly impossible to recover if this is the case.". How thats not asking about spin recovery I do not know.

@Jamiec I think the image on the linked wikipedia page clearly distinguishes the intended difference in meaning between "spiral" and "spin". The spiral is just steady controlled banking descent into terrain, not noticed because of the vestibular system being tricked (and not paying enough attention to/believing the instruments).

@llama A spiral in a typical airplane tends to be anything but "steady"- without counter-spiral control inputs most airplanes will exhibit ever-increasing bank angles and airspeed. The disoriented pilot's natural response to seeing decreasing altitude (namely, pulling up) aggravates this tendency. A graveyard spiral often ends in an in-flight breakup rather than CFIT.

@Jamiec Spirals and spins are very different flight conditions and require very different recovery techniques. In a spiral, neither wing is stalled and recovery is as simple as leveling the wings with the ailerons and then bringing the nose up without overstressing the airframe. Trying to level the wings with the ailerons in a spin will just aggravate the spin.

Ok, I think you're right I didnt distinguish a spin from a spiraling descent. So yeah, mea culpa.

Pilots do not 'tend' to believe our instruments - we are trained to believe them. We have to, because our senses are not reliable in such situations. Yes, if you follow your gauges and ignore what the seat of your pants is telling you, such a situation is completely and easily recoverable, provided you have enough altitude. But following your gauges is also the best method for not getting into such a situation to begin with. That's what instrument flying training is all about, and even pilots who have no intention of ever flying in bad weather should get at least a little. It can easily save your life.

Yep. Though to be totally fair, there have been a number of airline accidents and incidents that happened (at least in part) because in a moment of stress, the pilot flying didn't believe their instruments and tried to trust their feelings instead. Instruments can fail, but they fail a lot less often than the human body.

I think this happens far more often in general aviation than the airlines, although I do not have actual statistics at hand. But airline pilots are pros, and regularly undergo training. GA pilots often get into trouble because they do not have such training, or what they have is seriously out of date. This is an extremely easy situation to avoid, IF you're clear and current on procedures.

Oh, no doubt. It's not common, but it has happened more than a couple of times over the last 40 years, and it's usually on airlines that already have poor oversight, poor training budgets, and/or bad pilot culture. Removing the support structures that would help prevent foolish mistakes.

Are you sure? I follow aircraft accident reports, and do not recall ever reading about an airliner having such an accident. All sorts of other things, to be sure, but my impression is that graveyard spirals are exclusively the province of GA.

What? I've not made any claims about graveyard spirals happening in airliners. I said there have been a number of airline accidents where the pilot was trusting their feelings instead of their instruments.

Well, the subject was graveyard spirals, and you wrote: "... but it has happened more than a couple of times over the last 40 years, and it's usually on airlines...", so I assumed you were still on the original topic. Regardless, HAVE you ever heard of an airline pilot doing that? I do not recall hearing of such an episode, ever.

The "it" in that sentence was referring back to my first comment.  I said sometimes even professional pilots don't trust the instruments, you said you think it happens more in GA than airlines, and I said that's true but it does happen. I'm sorry but I don't see the confusion.  No, I'm not aware of an airliner going into a spiral in specific, but I know there have been specific incidents of somatogravic illusion leading to crashes or near-crashes. There was an Afriqiyah Air in Libya in 2010 and a Gulf Air in Bahrain in 2000, for example.

So you misunderstood - I was talking about the spiral the entire time. My 'it' was the spiral - nothing else. No big deal, just one of the things that often happens in written-only communication.

Making life-or-death decisions relying only on electronic instruments that may be erroneous when your whole body is telling you otherwise is nearly impossible, I think. Or am I wrong? Do pilots tend to believe in their instruments more than their own body's senses?

This is a required part of even private pilot training. The student's view outside is covered, and then the student is made to lower their head or close their eyes while the instructor does their best to confuse the student's vestibular system and then leaves the airplane in an unusual flight attitude. The student then needs to recover to normal flight using only their instruments. This is emphasized even more in instrument training.
Instruments can be wrong, but they are much, much more reliable than the pilot's vestibular system, which will fail very quickly once visual references are gone. Still, the fact that they can be wrong is worth addressing. The instruments required for IFR flight are redundant by design. In instrument training, pilots are trained to cross-check their instruments to identify when one is giving erroneous results and continue flying safely when one or more instruments have failed.
This training is not always enough. While "partial panel" training is a large part of instrument training, real life partial panel situations with minimum equipment in actual IMC often end badly. Airplanes used in the airlines have additional redundancy in their instruments to keep any single point of failure from putting the pilots in too difficult of a situation. All of them have at least two attitude indicators, and most of them have three.
In one sense, the actual recovery from a graveyard spiral is not especially difficult. Simply level the wings with the ailerons and push or pull as needed. The airplane will naturally pull up as it seeks its trimmed angle of attack. At higher airspeeds it will do so too aggressively and risks overstressing the airframe or stalling unless you push to unload the wings.
In another sense, it's very difficult. If you got into a developed graveyard spiral, it's probably because you were disoriented and not trusting your instruments. Realizing you're disoriented, reorienting yourself off your instruments, and recovering in what little time you have is not an easy task.

Yes, it is possible to recover.  I wanted to add more context as well.
I once helped a primary flight student recover from a graveyard spiral initiated during a steep turn maneuver in visual conditions.  There's more going on here than just instruments and vestibular feedback.
A graveyard spiral is also distinctly characterized by mechanical feedback.  When the airplane is in a descending turn, one of the most intuitive reactions is to try to raise the nose of the airplane by pulling on the control yoke.  However, beyond a reasonable angle of bank, attempting to raise the nose of the airplane makes the spiral tighter and worse in all respects.  This part was mentioned in the Wikipedia article but it was not explained very clearly.
In other words, the "stick and rudder" skill level of the pilot contributes directly to the entry and development toward CFIT or to recovery from this maneuver.
Yes, pilots are trained to recognize the effects of "load factor" in a variety of aircraft attitudes, to include the phenomenon of a spiral descent.
How it's done, in zero visibility, is primarily by recognizing an abnormal angle of bank (attitude) and rate of turn (heading), which should be understood as a need for decreased bank by rolling in the opposite direction of the turn.  This is a component of the exercise known as "unusual attitude recovery."
While electronic instruments can have a variety of faults and failure modes, they are generally extremely reliable.  Pilots learn that a single point of failure should affect only one instrument or a small subset of related instruments.


text:
Would it be feasible to use graphene in the gearbox of a large turbofan engine?
What is the possibility or are the drawbacks of substituting graphene for metal in a gearbox of a geared turbofan the size of a Trent 900, given its strength properties(graphene), harder than a diamond, cheaper than a diamond and 10 times the strength of steel?  Previous answers here, on the reasons why large bypass turbofan engines cannot be fitted with gears for efficiency have alluded to torque vs metal gear strength with metal not being strong enough and net energy losses to heat

In what form is graphene harder than diamond?

10 times the strength of steel in one direction only doesn't help much if the stress is in a different direction.

Well, just because it is stronger than steel does not mean that it is a useful structural material.  For example, the tensile strength of spider silk is higher than steel, but no one is building gearboxes out of spider silk.  For a graphene gearbox, the big challenge as I see it: graphene by definition is one atom thick. You aren't going to build a gearbox which is one atom thick. You can stack multiple sheets together to get something thicker, but the sheets will just slip against each other, so not useful as a structural material.  i.e. you can build a gearbox out of a big chunk of solid aluminum, but nobody tries to build a gearbox by taking 10,000 sheets of aluminum foil and laying them on top of each other.  What you would need is a graphene composite.  E.g. like we make carbon fiber composites today, which consists of carbon fibers held together by a polymer matrix.  Graphene composites are an active area of research, but I don't think they have reached the point where aerospace companies are considering them.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

sort: votes


text:
What are the pros and cons of adding a gearbox to larger jet engines?
What are the merits and demerits of adding a gearbox to larger jet engines like the Trent 900.  If its about torque, why not use a magnetic gearbox?  What are the demerits of using magnetic gearboxes in a geared turbofan engine?

Related

Its another thing to break and mechanic time is expensive....

Related too: What are the limitations in adoptions of GTF (Geared Turbofan) technology?

And for magnetic gearbox, you already mentioned this in Can a turbofan maintain torque when spinning fast?

Heavy, expensive, high maintenance, unreliable. You always get one or two of them.

The point of the gear box is not about torque but bypass ratio. With modern turbofans part of the means by which efficiency is achieved is to send as little of the air through the engine core as possible. This leads to dramatically different rotation rates required to achieve the proper blade tip speeds in the low pressure compressor and at the fan. Adding a gear box allows both the fan and low pressure compressor to operate at optimal speeds.

As I read it, the comment in the OP about torque is about the gearbox being able to handle the torque produced by the engine, not increasing the torque to drive the fan.

Low pressure compressor vs fan can be addressed by using seperate spools. Afaict the real issue is turbine vs fan.

@PeterGreen Adding more spools adds more weight and may require more turbine stages depending on how well they are able to divid up the power. Turbines are fairly flexible with the angles and speeds you can give them, unlike compressors which are much more prone to stalling.

One major benefit is that gearing slows down the fan
tips below supersonic, giving a major reduction in noise and drag.  The newer massive turbofan jets are ever improving in efficiency, and, with gearing, are taking a page out of the turboprop book to do it.

The goal is actually the opposite. A turbofan engine is unlikely to be designed with supersonic fan tip speeds because of the losses (the odd experimental usage aside). Rather, since the fan tip speed is limited to below the speed of sound, adding a gearbox allows the compressor speed to be increased instead, or maintained as the ratio of fan to compressor diameter increases.

You are are saying the same thing I am, the gearbox allows for slower fan speed. Slower bigger "wings" are more efficient.  Geared turbofans were developed by the British for noise abatement as well.

BTW the British engine was developed from a chinook (helicopter) turboshaft!

@XRF.  Not quite. The turbine needs to spin fast to be efficient, the fan needs to spin slowly. Adding a gearbox enables both to be optimised, rather than both (partly) comprised. So, it’s about the fan/turbine speed ratio, not the fan/compressor speed ratio.

@Penguin The compressor and turbine are already connected and their common diameter means their speed requirements are similar. The turbine design requirements are a lot easier on aerodynamics than the compressor and if it is any part of the turbine you want to run fast, it is high pressure turbine not the low pressure turbine, and that is because of weight savings not basic efficiency.

@RobertDiGiovanni You state that the fan speed is variable, that is not the case, it is limited to a fairly small range. It is the compressor speed that is being changed by adding a gearbox, not the fan, or rather in this case, the compressor is able to be built to a smaller size relative to the fan that would normally not be possible by allowing the compressor to spin faster.

Turboprop design is what I had in mind, essentially running the "fan" off the turboshaft the same way (ducted fan) using a reduction gear.  The design of the compressor/turbine would be up to you.  Lots of possibilities here, indeed, including reducing number of blades on the fan, no?

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
What is wrong about this interpretation of QFE and QNH?
I'm a little confused about the QFE and QNH definitions, and its application on aviation. I learnt that:
$QFE$ = pressure measured by an altimeter which is adjusted to ground level (it gives a height of 0 when the airplane is on the ground).
$QNH$ = pressure measured by an altimeter which is adjusted to sea level (it gives the field elevation when the airplane is on the ground).
So, by those definitions we can say that on ground level $QFE=p_0=101325Pa$ and $QNH=p_{ISA}(h_e)$ where $p_0$ is the SSL pressure and $h_e$ is the field elevation. $p_{ISA}$ is the pressure on a standard atmosphere associated to the height $h$. For the troposphere we have:
$$p_{ISA}[Pa]=101325\left(1-\frac{0.0065 h[m]}{288.15}\right)^{5.25588}$$
So, pressure drops with an height increment. For example, if the field elevation is positive (i.e. it's above the sea level) and the airplane is above the field we have to have $QFE>QNH$, because the height measured by $QFE$ configuration is lesser that the height measured by $QNH$ ($h_{QHN}=h_{QFE}+h_e$ and $h_e>0$)configuration, and by the relation between $p$ and $h$ if $h_{QFE}<h_{QNH}$, then $p_{QFE}>p_{QNH}$.
What I have seen in many problems of QFE determination by QNH and vice versa, is that the definitions used for QFE, QNH were exchanged. See for example this question How can I calculate QNH from QFE?.
I've seen this type of resolution done in many exercises, and so I presume that I'm wrong! But I still didn't get it! What am I missing here?
I can even show you a figure that was used on the theoretical slides of my Aircraft Performance course:

As you can see this figure says that $h_{QFE}=h_{QNH}+h_e$ which should instead be $h_{QNH}=h_{QFE}+h_e$ (the opposite) according to the $QFE$ and $QNH$ definitions.

Your question is quite not clear, you are challenging definitions and use of definitions, but you may start with a misunderstanding:

QFE = pressure measured by an altimeter which is adjusted to ground
  level (it gives a height of 0 when the airplane is on the ground).

Definitions
To ensure clarity, QFE/QNH:

are reference pressures, not altitudes
are not read/measured by the altimeter, but entered by the pilot in the setting window.
when mentioned in an altitude would be mentioned like, for instance: "I'm flying at 1,000 ft referenced to QFE", and not "my QFE is 1,000 ft".

QFE and QNH are not what the needles of the altimeter indicate, they are only the pressures that are to be entered in the setting window to calibrate the altimeter for common low level flight references.

How is QNH determined
While QFE is a direct measurement of the pressure, where the temperature is ignored, QNH is never measured, it is "retro-computed" from QFE:

Determining QNH requires first to know the local pressure (QFE) at the relevant location (e.g. airfield reference point)
The actual altitude of this location is then converted into a pressure difference, using the ISA model (+1 hPa per 27 ft at low altitude), without taking into consideration the temperature
The offset (which is positive unless the airfield altitude is under sea level) is added to QFE, the result is QNH (hence QNH pressure is usually > QFE pressure)

Said otherwise: QNH is the theoretical pressure value at MSL. but as the temperature wasn't taken into account, and actual distance between isobaric surfaces vary with actual temperature, then the actual pressure at MSL (if it could be measured) would be likely different.

Altimeter readings
After calibration with QFE or QNH value, any change in actual pressure -- that can be due to an actual altitude change, a local temperature change or a local pressure change -- will be sensed by the altimeter, the pressure change being indicated by an altitude change at the ISA rate of -27 ft for each added hPa.
By principle when the altimeter is on an isobaric surface equal to any pressure entered in the window, the value read is zero. As by definition the QFE is the pressure at the airfield level, then if the altimeter is at the airfield, set to QFE, and QFE has been just updated, then it indicates 0 altitude above ground level (AGL).
More generally (I hope this is accurate, else correct me):

Note that the pressure scale on the left has the higher value at the bottom, while the altitude scale on the right has it at the top.

As you can see this figure says that $h_{QFE}=h_{QNH}+h_e$ which should
  instead be $h_{QNH}=h_{QFE}+h_e$ (the opposite).

You or the book may be correct, depending on the sign of $h_e$. But it's not clear. If this quantity is positive, you are correct.
Remember that QFE and QNH are not altitudes, they are the altimeter reference values. For an airfield above MSL, QFE < QNH and an altimeter reading (h) referenced to QFE ($h_{QFE}$) will be approximately the height above the airfied (AGL) while when referenced to QNH ($h_{QNH}$) it will be approximately the altitude above MSL (AMSL).
Indeed altitude above sea level is larger than height above ground. Possibly in your example above, the author may have $h_e$ as a negative value.

Your question is a bit confusing since you seems to be already giving the definition for QNH and QFE.
I'll just leave this example to see if it can clear things up for you:
Let's say you are at an airfield which is at 500 feet above sea level. The ATIS tells you that the QNH is 1020hpa. While on the ground, if you set your altimeter to 1020hpa it will read 500ft, which is you altitude AMSL. Now if you want to know the QFE, just set your altimeter to read 0ft and you'll see the QFE on the altimeter setting window.
To wrap it up, setting the QNH on your altimeter will give you your altitude AMSL and the QFE your altitude AGL.
I hope this helps.

As far as I know the altimeter uses $p_{ISA}$ to compute the altitude. If we are at 500 ft, to make the altimeter say "500 ft" we would need an input pressure lesser than 101325Pa (at the SSL pressure, the altimeter says "0 ft"). So how is it possible to QNH be 1020hpa (which is greater than 101325Pa) when we are at 500ft? That's my question.

1013.25hpa is the standard pressure at sea level (like when we say the standard temperature is +15°C). However in real world the pressure is constantly changing, so you can have a pressure of 1020hpa or higher even if you are at sea level.

So the altimeter doesn't use $p_{ISA}$ to compute the altitude?

@ÉlioPereira Let's say that at airfield that is known to be 500' MSL and there is a dry well there that is 500' deep. With your altimeter in hand, you descend (long ladder) to the bottom of that well. You're now at MSL. You set your altimeter to zero feet. The barometric setting in the Kollsman window will now show the current QNH setting for the airfield.

I thought that an altimeter used $p_{ISA}$ to compute the height. So if we needed the altimeter to say "0ft" we would need to put at the input a pressure of 101325Pa

@ÉlioPereira The typical altimeter doesn't do any computation in the sense of using a microprocessor. It just has a knob. When you rotate the knob, both the altimeter hands and the pressure setting move. It's just a matter of linked gears. Increasing one of the settings decreases the other, and vice versa. You can choose to set either.

@ÉlioPereira A bit more info that may help. Apologies if it's too rudimentary. Both the altimeter hands and the pressure setting in the Kollsman window change when you rotate the knob. However, only the altimeter hands change when you actually change your altitude; the Kollsman window remains the same.

An altimeter that uses a microprocessor would measure the pressure altitude (1013.25 kPa or mbar, 29.92 in Hg) then adjust for the offset imposed by the baro setting. The baro altitude is what gets displayed on the actual dial.

my above comment should be 1013.25 hPa, which is equal to mbar


text:
Can pilots hear their passengers clapping on touchdown?
Something that just popped into my head: I've been on a few easyJet and Ryanair flights where a lot of passengers clap and cheer on touchdown. Would the pilots be able to hear this?
Here's an example I found by searching YouTube:

It seems pretty commonplace... but can the pilots hear them? I guess it would be distracting.
Just something I was wondering!

i can testify that i've heard the 'reverse' happening- the sound of the master warning of the autopilot turning off in the first few rows :)

Yes, but the question is "Can pilots hear clapping?"

I cannot understand how the chatterboxes in that plane even noticed that the plane has landed.

In my 10 years of flying 747s on international flights, there was no way we would know of passenger reactions upon landing unless one of the cabin staff told us, and that was rare.

Yeah, I would imagine that to be the case in a 747 due to where the cockpit is. I suppose that the answer to this question is really "it depends on the aircraft".

We can in my airplane:

Quite frankly, I'm usually a little insulted when it happens.
Were they actually so fearful for their lives and are now so happy to be alive that they need to applaud the heroic pilots?  Sorry, but unless it's a true emergency and we did actually just save their lives, I'm not putting them in a situation where they are actually at risk.  In this case, I was just doing my job.  Do you applaud the waiter or waitress who gets your order right??  :-)
As far as airliners, I'm pretty sure that the pilots can hear you too if everyone is clapping but someone with experience on that should confirm whether or not this is true.

Disclaimer:  This isn't the actual airplane that I fly, but it is the same kind.  Notice also that we don't have a door to our cockpit so this makes it much easier to hear what is going on in back.

I don't know, most people thank the bus driver when they get off in the UK :) ...I don't understand it either though!

@DannyBeckett Thanking is one thing, applauding is another!  :)

That's a fancy airplane!  Does it always come with a glass of wine :)

@KORD4me It actually comes with a few bottles of wine along with other liquor!  :-)

@Lnafziger Haha!  I like the disclaimer.  You had me going tho, thought for sure it was YOUR airplane.  Jealousy was raging!

@KORD4me Ahh, no I don't own one, but I do fly one just like this!

I also love the way passengers clap as soon as the wheels touch the tarmac. I'd love to know if the passengers on this flight were applauding the pilot as the plane ran off the runway onto a highway.

I'm most of the owners (with some exceptions) of planes like those don't bother to fly them.

@Farhan True, but I know that Jimmy Buffet flies his!   :-)

@Lnafziger Yes, that's why I wrote "with some exceptions". Larry's planes must be like that too.

I imagine part of the reason people clap is out of respect for pilots who operate large, complex machines ... and make it look easy!

@Lnafziger, I suspect people usually clap based on the perceived 'smoothness' of the landing. Or maybe if there's been some scary 'event' before (turbulence or a go-round).

A rock band, for instance, must play really badly not to earn any single applause. I think, all crew is rewarded for a flight as such, stewards including.

I never clap myself but:

Flying is perceived by some to be a dangerous and unnatural activity, people clap due to relief at still being alive.

Lots of people on Ryanair flights probably only fly a couple of times a year so it's not a regular event and they are likely to be in high spirits as they are going on holiday.

Re spirits, they are likely to have consumed a few of those too.

Ryanair encourage this type of behaviour by playing a fanfare when they arrive "on time" and this is probably perceived by some passengers as being a fanfare to say "hey, we didn't die!".

Musicians or actors who get applauded are also “just doing their job”.  If I applaud you, it doesn’t mean I thought you might fail or have difficulty, it just means you have done something that I find impressive, usually because I would find it very difficult.  Isn’t it OK to see flying that way?

Frankly nowadays a good waiter/waitress would deserve applause just to separate them from their sometimes incompetent colleagues. But that is a different world, instead of applause they get a tip in order to compensate for their mostly mediocre wage...

Depending where you are, university students often applaud after the end of a good lecture by a professor (probably some clapping for the lecture and some clapping for the end), but not because they thought they were in danger.

@PeterLeFanuLumsdaine Well, a performer's job is to entertain, and that clapping is to give them feedback on how the did....  I suppose one could say that the passengers are giving feedback to the pilots as well, but it doesn't usually happen in other similar situations (busses, taxis, etc.)


text:
Why don't all flights use the North Atlantic Tracks (NATs)?
I had assumed that all flights between western Europe and North America would use the North Atlantic Tracks (NATs) but this answer says that, actually, only about half do.  What reasons might a flight have for not using the tracks?
Wikipedia says that Concorde didn't use the tracks because it flew above them but I'd expect all commercial transatlantic flights these days to be within the band covered by the tracks (FL290–410).

If you read that answer it explains that not all traffic in the North ATlantic crosses between Europe and America. It also gives examples of flights over the NAT that don't use the Oceanic Track System.

What you abbrevintend as NAT is OTS in that answer.  NAT means North ATlantic in that answer. That is perhaps where the confusion comes from.

I don't know what the current system entails, but prior to my retirement in 1999, you had to have additional certification for your aircraft, your pilots, and your check airmen over and above what was normally required. Additionally you had to follow procedures that you would normally not do. All of that cost money, so for many the additional cost was not worth the benefit.

Many of the crossings are done in smaller jets (Gulfstream, Falcon, Bombardier) which can take advantage of higher altitudes and so do not use the NATs.

The answer doesn't state that half of the flights between western Europe and North America use the North Atlantic Oceanic Track System (OTS). It states that half of the flights over the North Atlantic use the OTS.
Part of the flights over the North Atlantic don't cross between western Europe and North America and they don't use the tracks. Examples are flights to/from Iceland or the Azores. Also flights crossing between Ireland and the North Western point of Spain are entering the North Atlantic Region.
Also part of the flights that do cross between western Europe and North America and vice versa may use random routing for various reason.
On some days the winds are such that favourable tracks to North America are within the Reykjavik control area, which allows for random routings.
Some combinations of origin / destination are better served by avoiding the OTS area at all. For example flights between the Iberian peninsula and Florida are generally South of the OTS.

I think improvements in aircraft performance has a lot to do with it. Business jets regularly cruise at FL430-450, and newer model airliners are now scraping the low 40's. If you can climb above the tracks, random routing can be a real advantage.


text:
Could a plane be constructed to be fly in fixed-stick roll-stable circles?
According to https://aviation.stackexchange.com/a/12511/973 it is not possible for a plane to aerodynamically recover from a roll disturbance.  Is that an inherent limitation of how planes could be constructed, or is that merely a design trait that is accepted in exchange for other considerations like fuel economy and ease of control?  Would it be possible to construct a plane such that if the controls were eased into a certain setting, the plane would settle into a stable spiral climb until it reached an equilibrium altitude set by the thinning air, and such that mild disturbances in the roll axis would settle out [possibly changing the plane's heading]?
I would expect that maintenance of roll stability during straight and level flight would be complicated by the fact that roll won't immediately affect the flow of air over a plane in the same way as would a change in pitch or yaw.  If a plane was flying in deliberate circles, however, I would think that a change in roll would rather quickly change the rate at which the plane would want to rotate about the yaw axis, and that aerodynamic surfaces cause such change to produce torque about the roll axis, thus counteracting the disturbance.
My intuition would suggest that even if roll stability could be achieved, a plane designed for such stability would probably be annoying to fly in normal usage.  If, however, it was possible to activate a control which would induce such stability, it would seem such a thing might be helpful in some situations where a pilot might otherwise be in trouble [e.g. unexpected sudden IMC or other conditions causing loss of visual horizon reference].  Would such a thing be aerodynamically impossible, theoretically possible but practically unworkable, or workable but not insufficiently useful to be worthwhile?

Hydraulicly boosted ailerons helped the P-38 Lightning.

Maybe with a cable tethering it to the center of the circle.

Paramotors are intrinsically stable. Just put the wing much higher than the CG.

@KevinKostlan: Care to expand that into an answer?

This will become another of my long posts; after all, lateral stability is more complex than longitudinal stability and involves many more factors. Short answer: It is possible, but not by aerodynamic means, and would incur a drag penalty.
Starting condition
Assume we have an aircraft with fixed controls, trimmed for straight flight, and make it a glider to remove propulsion effects. Next, let it fly through an asymmetric gust which lifts one wing. For what follows we assume calm air again.
Initial sideslip
For an outside, earth-fixed observer, a component of lift is pointing sideways and not compensated by weight, so the aircraft will accelerate to that side. From the point of view of the aircraft, lift is still acting in the plane of symmetry, but gravity does not and will cause it to sideslip. This will mainly evoke these reactions:

Directional stability $c_{n\beta}$: The aircraft will yaw into the wind because the vertical tail will create a side force ("weathervane effect").
Dihedral effect $c_{l\beta}$: The additional lift on the windward wing will lift this wing. Also, the additional side force on the vertical tail, which yaws the aircraft, will cause a supporting rolling moment. Roll damping ($c_{lp}$) will limit the amount by which the aircraft rights itself up, however.
Yaw moment due to dihedral and wing sweep: The lift difference due to dihedral and wing sweep will retard the upwind wing, causing a yawing motion into the wind. This effect can be added to the directional stability $c_{n\beta}$.

Eventual (almost) coordinated turn
The aircraft will start to fly a coordinated turn, because all forces try to drive sideslip to almost zero, and on their way to do so start a yawing motion. In contrast to that, the roll angle will not change further once sideslip stops, because the sideslip-induced rolling moments stop as well. However, if a roll angle remains, the aircraft will now begin to turn. This opens up a new bunch of effects, because  now we have asymmetric flow: Due to the yawing motion, airspeed varies over wingspan and sideslip angle varies over length:

Rolling moment due to yawing motion $c_{lr}$: The outer wing (which was the leeward wing during sideslip before) now travels faster, creating more lift. This causes a rolling moment which increases roll angle.
Since the angle of attack would be the same in a coordinated turn over the whole wingspan, drag will behave proportional to lift and increase with the local turn radius. This drag difference will create a small yawing moment which will let the inner wing advance, until the dihedral effect will kick in and create enough of a difference in angle of attack that the drag on the inner and on the outer wing are equal again. Since induced drag is proportional to the square of the lift coefficient, the wing will still create less lift on the inner wing than on the outer wing, but the difference will be reduced. If the airplane climbs or descends during the turn, angle of attack will vary over span and will create an uprighting moment on the descending airplane (and vice versa).
Yawing moment due to yawing motion $c_{nr}$: This is also called yaw damping and creates a yawing moment opposed to the yawing motion. Contributing factors are the vertical tail which sees a sideward flow component due to the yawing motion, and the drag distribution along the wingspan. Aircraft with a long tail have high yaw damping and, in combination with plenty of dihedral, tend to be more roll stable than short-tailed aircraft. Here the long tail forces the wing into a sideslip which uprights the aircraft. This configuration is found on free-flying models which do indeed upright themselves from shallow banks.
Centrifugal forces: The sideward component of lift is now balanced by the centrifugal force due to the turn. However, the centrifugal forces also will create an uprighting rolling moment which depends on the spanwise distribution of masses.

Note that the outer wing has a bigger turn radius $R$, but the same angular velocity $\omega$ as the rest of the airplane. This causes it to experience more centrifugal force $m\cdot \omega^2\cdot R$ than the inner wing (symbolized by the length of the parallel arrows). I used a rather steep attitude to get the point across, but the same holds true at shallower bank angles. This is the uprighting inertial moment.
Depending on the relative size of these effects, the aircraft will either right itself up, stay at this roll angle or dive ever deeper into the turn. Agile aircraft with low roll inertia will have too little uprighting moment and are likely to spiral dive. More stable configurations with big vertical tails and ample of dihedral will either continue to turn gently or right themselves up.
Effects at non-zero roll rates
Now this was all done under the assumption that the roll angle changes very slowly. A too small vertical tail and/or too much dihedral will excite the Dutch roll motion, and now the roll angle oscillates, adding more roll-induced effects. To keep this answer reasonably short, I will not list them here.
Sweptback configurations optimized for fast flight (= with small vertical tails close to the wing) are typical examples where the ratio $\dfrac{c_{l\beta}}{c_{n\beta}}$ is too high and the Dutch roll eigenmode has too little damping. Free flying model aircraft, on the other hand, cannot afford to fall into a spiral dive, and by giving them big vertical tails and long lever arms, which allows them to have substantial dihedral, and a high radius of roll inertia, they can be made to upright themselves.

As a point of clarification with regard to your last paragraph, are you saying that aircraft can be designed with a significant "basin of stability" (meaning the apparent tendency of such models to settle into stable flight is genuine), but such designs are too inefficient and unmaneuverable to be acceptable in full-sized aircraft?  Could a pilot control system which had no visual, gyroscopic, or other reference for "up" recognize the oscillation pattern of a dutch roll and act to damp it and restore stability, or would apparent g forces remain too close to constant?

@supercat: Historically, airplane designers have preferred the unstable, less draggy way and left it to the pilots to stabilize the plane. This made a gyro reference necessary when visual cues were gone - gs alone would not help. I wouldn't go as far as saying that more stable designs were unacceptable - they were not attractive enough to become the standard. Many successful aircraft had a weakly damped dutch roll and tended to "wag their tails" during flight, but the axis of rotation was close to the cockpit and the amplitude small, so the pilots never bothered.

I just remembered another thought: would it be possible for someone to fly a plane using just compass, airspeed, and altimeter, and contact with radar operators (who could notify the pilot of minimum safe altitude for terrain and ensure the airspace in front of the plane is clear)?  I would think that if a pilot tried to alternate climbing and descending while watching the compass, the ride might be uncomfortable, but any significant roll angle should make itself apparent in the compass heading.

@supercat: This needs a lot of concentration - the compass needle is aligned to the magnetic field lines, wich have an inclination versus the horizon on all latitudes except the equator. Thus, when the aircraft starts rolling and circling, the compass will not show the proper heading, but lead or lag, depending on heading and circling direction. The compass is uniquely poor for short-term heading measurement, but with some fancy calculation this could be remedied.

@supercat I've seen video of a Cessna 120 being flown in this manner, with the turn rate indicator covered up. A steady descent was flown through over a thousand feet of overcast.  A key point is that in the northern hemisphere, a heading very close to magnetic south works best because the compass error exaggerates any bank and this gives an advance warning of what is going on.  A northerly magnetic heading is worthless. The pilot was making control inputs primarily with the rudder, using the planes slip-roll coupling to effect changes in bank; this system tends to avoid overcontrolling.

@supercat The pilot was making control inputs primarily with the rudder, using the planes slip-roll coupling to effect changes in bank; this system tends to avoid overcontrolling, especially when flying by reference to the compass, compared to more "coordinated" control inputs.

@PeterKampf I think there is a problem with the theory expressed in your answer, and that is the theory that sideslip tends to go to zero over time even if the plane remains banked.  There are aerodynamic reasons why a banked turn tends to involve some sideslip unless a rudder is used to prevent this.  These effects are most pronounced at low airspeeds.  Free-flight model airplanes could not exist if your theory were correct-- any turn would end up winding up to a very steep bank angle due to the increased airspeed, and lift, experienced by the outboard wingtip.

@PeterKampf -- due to these dynamics involving sideslip, there are many situations where dihedral can bring a plane to wings-level flight after a disturbance.  A plane like a "Gentle Lady" RC glider will remain at an average bank angle close to zero for many hours even with no pilot input, even in air with some turbulence, and that's not really compatible with some of the ideas expressed in this answer.

@PeterKampf furthermore the description of the cause of initial sideslip is wrong.  Just pointing out that gravity has a sideways component in the plane's reference frame isn't really sufficient to explain why this should drive a sideslip-- the same is true in a coordinated turn.  Ultimately, any theory of why banking should cause a transient initial sideslip is going to come down to a discussion of yaw rotational inertia-- a temporary delay in establishing a required yaw rotation may be viewed as a cause of sideslip.

@quietflyer: Strange that you can only criticise and never bother to give a better answer. What else but gravity is out of symmetry at a banked state without turning motion (= no centripetal force!)??? Besides, your modelling experience should tell you that the stable models have lots of dihedral and a comparatively small vertical on a very long tail. This means lots of yaw damping with comparatively little weathervane effect. This helps them to right themselves up. Maybe I should improve that part.

@PeterKampf -- the emphasis on inertial uprighting forces in this answer is interesting-- but I still think it misses aerodynamic effects that tend to cause sideslip whenever an aircraft is banked and turning,  For example even if the vertical fin is fully streamlined to the curving flight path, the wing cannot be-- the wing will experience some sideslip.

@PeterKampf -- " What else but gravity is out of symmetry at a banked state without turning motion (= no centripetal force!)??" -- fundamentally, force causes acceleration, not motion.  If the aircraft is tipped into a bank but is still generating only 1 G of lift, the net force and acceleration will be both sideways and downward.  If the aircraft is tipped into a bank and the wing "loads up" (due to pilot pitch input or airspeed gain) to the appropriate G-load for the bank angle, then the net force and acceleration will be purely sideways.

@PeterKampf --  I would submit that the latter case involves more initial sideslip than the former case, and in both cases the initial sideslip can be viewed purely as the result of yaw rotational inertia needing to be overcome  before the nose starts rotating around the horizon in synch with the curving flight path.  After all a sideways acceleration is nothing more than a curvature in the flight path.

@PeterKampf --  I'd also submit that aerodynamic-driven effects related to turn rate are more important as drivers of sideslip in the long run, than this effect of yaw rotational inertia that comes into play when we abruptly bank the wing.

This really is the first time I've ever heard anyone assert that a high moment of inertia in the roll axis tended to contribute a self-righting effect.  Interesting-- if true.  But is it?

Let's keep in mind this article is more than 4 years old and contains many interesting and well illustrated points.  One in particular is the righting effect of the vertical stabilizer in a side slip.  Notice tall fins were the rage in the early 1950s, while "chunkier" ones were dominant in earlier designs. A tall fin, for its area, will create more rolling torque in a sideslip, leading to a condition of slipping one way and rolling the other. The lower aspect "chunky" tends to act more as a weathervane, promoting an (easier to get out of) spiral.

This is not an answer to your question and I am guessing there are some terminology issues here (for example, what does 'aerodynamically recover' mean) but
it is certainly possible for some aircraft in some configurations to have stable roll dynamics.
Different aircraft have different dynamics, so partly the answer is it depends.
Furthermore, the dynamics depend on the operating configuration.
Aircraft lateral dynamics have some roll-yaw coupling but it is typically very slow.
For small perturbations in calm conditions the dynamics can be stable (as in the linearisation eigenvalues have negative real parts) but this can be illusory as the basin of attraction can be very small. This means that typical disturbances can pop you into an 'unstable regime' (I am using the term loosely here).
The lateral behaviour of the aircraft around an operating point can be typically characterised by three modes. One mode is what governs the aircraft's primary response and is typically stable and behaves reasonably. Another mode is called the spiral mode; it is possible that this is unstable, and in either case is typically very slow. (Model aircraft typically have a large dihedral to stabilise this mode.) The final eigenvector is a Dutch roll mode  which is
stable, but is slow and very lightly damped.
As a further illustration, an aircraft in stable flight can (typically, in reasonable conditions, etc, etc.) be piloted by elevator & rudder alone (the
linearised dynamics are completely controllable from the elevator & rudder deflection inputs). This would not be possible if these inputs had no effect on roll (because there would be no way to deal with unstable rolling).
As a bad analogy, consider riding your bicycle without hands. Typically it can be done, but it doesn't take much (bump on road, sneeze,...) to destabilise.

"Could a plane be constructed to be fly in fixed-stick roll-stable circles?"
Absolutely.  Consider a radio-controlled sailplane with lots of dihedral or polyhedral, such as the famous "Gentle Lady".  With the rudder set at a fixed deflection off-center, the aircraft will tend to stay at a given bank angle, and will tend to return to that bank angle after a disturbance.
The root of what is going on here is that there are aerodynamic effects that tend to cause sideslip in circling flight, especially in slow-flying aircraft, and the sideslip interacts with dihedral to tend to return the aircraft to wings-level after a disturbance.  That's for the rudder-centered case.  With the rudder deflected, the aircraft tends to return to a fixed bank angle.
These aerodynamic effects have to do with the "curving" nature of the relative wind in a turn.  Since the aircraft is rotating as well as translating, different parts of the aircraft are moving through the airmass in different directions at any given instant.  Even if the vertical fin were perfectly streamlined to the flow at any given instant, more forward parts of the aircraft-- including the wing-- would be experiencing some sideslip.
This is highlighted in this section of John S Denker's excellent "See How it Flies" website-- https://www.av8n.com/how/htm/yaw.html#sec-long-tail-slip
Some of these dynamics are discussed-- perhaps with a few simplifying assumptions -- in these articles by Blaine Beron-Rawdon in the magazine--"Model Aviation"-- a 2-part series of articles called "Spiral Stability and the Bowl Effect" (September and October 1990) and a series of 4 articles entitled "Dihedral, a 4-part series" (August through November of 1988).
"Spiral Stability and the Bowl Effect" series--
Part 1 http://library.modelaviation.com/ma/1990/9/spiral-stability-and-bowl-effect
Part 2 http://library.modelaviation.com/ma/1990/10/spiral-stability-and-bowl-effect
"Dihedral" series --
Part 1 http://library.modelaviation.com/ma/1988/8/dihedral
Part 2 http://library.modelaviation.com/ma/1988/9/dihedral
Part 3 http://library.modelaviation.com/ma/1988/10/dihedral
Part 4
http://library.modelaviation.com/ma/1988/11/dihedral
There are also transient effects -- yaw rotational inertia is one-- that will tend to cause additional sideslip immediately after an increase in bank angle.  These effects are generally less important than the aerodynamic effects described above, if we are interested in whether or not an aircraft will eventually roll back to wings-level (or to a given "trimmed" bank angle.)
You are right that roll stability is useful for cloud flying.  I've been able to maintain control of a sailplane somewhat like the "Gentle Lady", but made of EPP foam, while circling in cloud for quite some time with no visual contact.  I had telemetry of altitude and climb rate, which helped assure me that the plane had not accidentally fallen into a spiral dive -- due to aeroelastic effects this IS possible even with the basic built-in stability of such an aircraft.  In such  an event, my "escape route" was to enter an intentional spin or, an inverted turn with the stick full forward and full to one side.  Not recommended for full-scale aircraft.
Note that both the Gentle Lady and the similar EPP glider have low moments of inertia in the roll axis.
"My intuition would suggest that even if roll stability could be achieved, a plane designed for such stability would probably be annoying to fly in normal usage"
Yes, generally speaking that is true.  Roll control response to aileron inputs will be sluggish if even slightly less than fully "coordinated" w/ rudder, crosswind gusts will cause unwanted rolling, and there may be a tendency toward "Dutch Roll" oscillations in some parts of the flight envelope.

If there hadn't been an answer that cited model aircraft, I'd have had to write one.  I'd go further -- free flight models have been built to fly in circles with fixed surfaces since at least the 1930s.  Pretty well has to be "fixed stick" -- further, they used opposed wing warp and rudder (as well as thrust line) to climb turning one way and glide turning the other.

Out of curiosity, what would be the practicality of having some control surfaces that could be engaged to put an airplane into such a configuration if e.g. there was an unexpected loss of visibility, but would not interfere with normal maneuvering otherwise?

I think you'd basically talking about the ability to make the wings pop up to an abnormally large dihedral angle-- probably not very practical.

There can't be any such mechanism that works purely by aerodynamic means.
The reason for this is that, at least on a short timescale, the air doesn't know which way is up.
The airflow can feel deviations in pitch and yaw, because they result in the air coming at the aircraft from a different direction (relative to its structure). This gives an opportunity for the shape of the aircraft to be designed such that the altered airflow produces a righting moment.
However, if you imagine a deviation in pure roll, the air will still be coming at the aircraft from the same direction as before, namely from straight ahead. All of the air flows will be the same as in level flight, just with everything turned to match the roll angle of the aircraft.
There is the difference that if the wings are not level, the direction of lift is no longer opposite to the direction of gravity. In the not-completely-short timescale, this will cause the aircraft to accelerate with respect to the air. Eventually that will lead to flying at through the air at an angle (that is, changing the angle of attack and/or sideslip), which the aerodynamics can react to. I think this must be what Peter Kämpf meant when he said that there can be an "inertial mechanism" for roll stability.

One way is to take a plane with a high dihedral wing and low set CG and simply rudder it.
Important design aspect is to put the dihedral in the wing, rather than to try for uprighting effect from a tall tail for roll stability.  A large unsymmetric tail away from the center of gravity will promote yaw in a slip, which is destabilizing to the initial roll, creating a spiral.
You do not need a thresher shark tail to have a stable aircraft.
Why low set CG?  Here is your roll damper.  No need to be grossly pendulous, just a little low.


text:
How are unpaved strips created and maintained?
I'm under the impression that some large craft are rated for "unpaved strips", which appear to be either some sort of gravel or perhaps dirt.  The 727, and this runway, being a good example of this:

Picture taken from this question, which is also a good reference on how large airplanes can operate on "rough" fields.
What I'm wondering is how these fields are prepared?  I assume the ground is required to be able to hold up a certain amount of weight, and I also assume that ability must be uniform, or you'd be risking a wheel poking into the runway and basically ruining everyones day...
So how do they prepare/create these runways?  And how do they ensure that they maintain the ability to be used for operations with craft that are rated for "unpaved" but probably not rated for "random, somewhat grassy field."

Advisory Circular (AC) 300-004 from Transport Canada has detailed information regarding the maintenance of unpaved runways.

The purpose of this document is to outline methodologies for the
  measurement and reporting of surface shear strength for unpaved
  runways.  In addition, the document outlines recommended practices for
  condition inspection, maintenance and repair of airport gravel
  surfaces and turf landing strips.

Runways should be designed with consideration for drainage (slope, crown, etc), susceptibility to frost heave, vegetation, roughness, etc.
Maintenance can be based on surface hardness measurements made with a penetrometer. A schedule for periodic inspection is created when the runway is new. The AC recommends a maximum of three years between hardness measurements. One type of penetrometer measurement is described as follows:

The Boeing High Load Penetrometer (Reference Boeing Document No. D6-24555 - High Load Penetrometer Soil Strength Tester) consists of a hydraulic cylinder with a cone point test probe mounted at the rod end.  The hydraulic cylinder is normally positioned against the frame of a heavy vehicle which serves as a reactive load.  In the test procedure, the probe is driven at a steady rate to a 100 mm (4 inch) depth into the surface by the application of pressure through a hand pump.    Generally a pressure reading is taken about 30 seconds or less after movement of the penetrometer has stopped at the 100 mm (4 inch) depth.

Once maintenance is deemed to be required...

Gravel pavement surface maintenance primarily involves periodic
  grading to remove the surface irregularities developing with time and
  to re-establish grades for drainage purposes. Occasionally, new gravel
  has to be added to replace lost material.  Dust suppression measures
  may also be needed during the summer months.

The recommended aggregate sizes are shown in this table from the AC:

The recommended grading method is shown here:

Also note a wealth of information in the reference documents listed in section 2.1 of the AC.

This presentation (.pdf link) from a company that provides airport maintenance services has additionl useful information and photos.

First off all pilots are trained for soft field landings and I assume the procedures for large craft are not all that different from small craft. The main goal is to keep weight off the nose wheel as much as possible.
How the field is prepared will depend on where it is geographically. In the desert or other dry hard areas this is not really that hard for obvious reasons. In a grassy area you will need to mow the grass and possibly clear some trees or other low brush. The area of PA I fly in is littered with private grass strips that are hard to differentiate from the fields next to them. Many of them look like this

As for the uniformity of the ground beneath them, as far as I know they are not altered in any way. This factor is mainly altered by the weather. If it has just  rained the ground is going to be muddy and slick, possibly not useable. Check out this question for more info on that.

Interesting information, though mostly I'm wondering how this is handled for runways that deal with much much larger aircraft.

Dirt strips intended for large aircraft are typically only found in very arid areas, and preparing/maintaining the strip is a fairly simple matter of running a grader or bulldozer over it every so often, possibly followed by a heavy roller. That's still cheaper than trying to maintain an asphalt or concrete strip, especially in very sandy locales where wind erosion can bury a concrete strip in days. A grass strip, as in this answer, is found in more temperate areas to prevent soil erosion, and while a larger airliner might be able to set down here in dry weather it's never ideal.

@KeithS If you could expand that out and make it into an answer.  The situation you are talking about is the exact situation I'm curious about.

@KeithS Especially if you know of any equipment they might use to test to firmness of the ground...


text:
Do jet contrails impact weather?
Do jet contrails impact weather at the ground?

This seems like two separate questions - a fact-based assessment of whether the weather on the ground changes with the presence of contrails; and a question about whether a specific conspiracy theory is true or not that would be better suited for Skeptics (which they have already addressed).

The TL;DR is "Yes, but in a way that's useless to conspiracy theorists -- they're cirrocumulus clouds by another name"

During the 9-11 groundings, the skies over the US were reported to be unusually clear. As flights resumed, sky conditions returned to their previous patterns. The conclusion drawn from these observations was that contrails have a tendency to increase cloud cover via a sort of "seeding" effect.

Short answer
Based on past observations and a study led by Patrick Minnis from NASA, contrails could result in increasing the temperature over the ground by 1°C every 30 years over the US.
From a study by German researchers Ulrike Burkhardt and Bernd Kärcher, the effect of contrails is comparable in other areas with a similar level of air traffic.
Because the air traffic is said to be about to increase very quickly, the effects of contrails on the temperature may increase too.

Contrails and natural cirrus: So similar that studies are difficult
Contrails are like cirrus, and actually some just become persistent cirrus. Sometime it's difficult to tell which one is a condensation trail, which one is a natural cloud. This makes more difficult the study of contrails effects, and introduces many uncertainties that need to be reduced:

(source)
The current studies take into account only the young contrail-cirrus, because they are still relatively easy to differentiate from natural cirrus, due to their linear shape. Most aged contrail-cirrus are not differentiable and are not counted as aircraft generated. This means the current results potentially under-estimate the effects of contrails.
Pertaining to the discussion, contrail-cirrus as clouds:

Induce a cloud-feedback, i.e. the coupling between cloudiness and surface air temperature.
Induce a radiative forcing, they increase the portion of the energy not re-emitted to space. The quantity is expressed in mW/m².

Increasing the air temperature has a link with increasing ground or oceans temperature, but the two notions mustn't be mixed. However cirrus are suspected to play a role in the global warming, because they could introduce a positive feedback into this warming.

Study: Coverage and effects of contrail-cirrus
Global radiative forcing from contrail cirrus published in Nature in 2011 was a study to evaluate the contrail-cirrus coverage, and its impact. Conclusion:

Aviation makes a significant contribution to anthropogenic climate
  forcing. The impacts arise from [...] and from changes in cloudiness in the
  upper troposphere. An important but poorly understood component of
  this forcing is caused by ‘contrail cirrus’—a type of cloud that
  consist of young line-shaped contrails and the older irregularly
  shaped contrails that arise from them.

The estimated effect is depicted on this map (mW/m²) from the study:

Net effect of contrails (CIC) warming compared to other aviation factors of warming:

(source)

Study: Effect of contrail-cirrus on US climate
NASA conducted a study in 2004: Patrick Minnis, J. Kirk Ayers, Rabindra Palikonda, and Dung Phan -- Contrails, Cirrus Trends, and Climate.
This study was analyzed in Science Daily: Clouds caused by aircraft exhaust may warm the US climate:

Using published results from NASA's Goddard Institute for Space
  Studies (New York) general circulation model, Minnis and his
  colleagues estimated contrails and their resulting cirrus clouds would
  increase surface and lower atmospheric temperatures by 0.36 to 0.54 degrees Fahrenheit per decade.

Aviation radiative forcing in 2000: An update on IPCC (1999)
This study was included in a new one by R. Sausen et al. in 2005. It shown the radiative forcing by aircraft could have been slightly underestimated:

Source: Aviation radiative forcing in 2000: An update on IPCC (1999) - R. Sausen

The US surface temperature is increasing by 1°C every 30 years: I would say this is very significant for plants, life, water cycle, oceanic currents, etc.
Also look at @Simon good answer about the increase of the difference between night and day temperatures, by more than 1°C, during the 3 days civil air traffic was forbidden on September 2001. This increase seems to confirm contrail-cirrus limit cooling on night time.

Hmm... That estimation seems a little dubious, since total global warming from the mid 1800s to the present is estimated at around 0.6 degrees C, IIRC. 1C per 30 years would definitely be very significant, but it also seems very dubious (unless maybe they're talking about increasing average temperatures in a few particular places by that much, rather than global average temperatures.)

@reirab Note that the total temperature of the system doesn't change - the only effect is that heat from the atmosphere moves closer to the surface, effectively. It makes the surface temperatures higher, but the higher altitude temperatures lower.

@Luaan The total temperature does change since it is not a closed system.  If it were closed, I would agree but the Earth receives and loses an enormous amount of energy each day.  Change either of those (reflectivity or total albedo for incoming, temperature gradient and absorption for outgoing) and the total energy in the system changes (if "Earth" is the system you are talking about).

@Simon Yes, but that's not the kind of system the report is talking about - they did not monitor the global change (if any), just the local impact. We're really talking about the specific study mentioned in the answer (the 2004 one), not Earth's temperature balance as a whole. The absorption of IR in water doesn't differ between regular atmospheric humidity and outright clouds - the only thing that changes is the albedo, and that should be higher for the cloud-scenario. So it's colder up, hotter down. But maybe I'm just reading that wrong.

You should change the first paragraph to mention lowering atmospheric temperature as well. As is, it implies that contrails generally contribute to global warming but actually if you look at the numbers it actually counters global warming (it contributes to global cooling). The rest of the answer is excellent though

@slebetman: Thanks for the feedback. Which figures are you referring to when you say contrails may introduce a global cooling? So far I'm not sure of the effect as the overall cirrus coverage may not vary (less natural cirrus balancing contrail existence).

@mins: Sorry. I based it on a previous article I read and thought the overall effect was cooling (more heat loss in the atmosphere than gained on the ground). My memory was wrong. It appears that while low altitude clouds do cool down the climate high altitude clouds like contrails effectively insulate the atmosphere so the result is a general warming: http://arstechnica.com/science/2011/03/contrails-impact-climate-more-than-planes-carbon-emissions/

@slebetman: Ok, thanks for the link. I feel that this field is not well understood today, so conclusions may vary while an increasing volume of data is being analyzed.

In the days immediately following the Twin Towers 911 attacks when there was no air traffic, a study found that the difference between night and day temperatures was greater and that day time temperatures were around a degree higher.
The idea about conspiracies to modify weather and chemtrails is well and truly shown to be complete nonsense - just Google for it.
Post 911 study

So, there was a statistical evaluation of some temperature difference, and the statistics used "the days immediately following the Twin Towers 911 attacks", that is, at most 3 samples. Ok, I do not even need to know what the values were compared to. That's not even valid nonsense!

While the 'results' of the study seem expected (there are far less contrails in the CONUS at night than in the day, so the daytime shading effect almost certainly outweighs the nighttime reduction in radiative cooling,) I must agree with Volker that 3 days is hardly statistically significant when studying weather. Literally a single low pressure system would be sufficient to make far more difference than what this study found. In the portion of the CONUS East of the Rockies (i.e. most of it,) it's not unusual for temperatures to vary by 20 C from one day to the next.

The exceptions to the above regarding daytime vs. nighttime air traffic would be Memphis, TN and Louisville, KY. Now, if a study found that the temperatures near those two places exceeded regional expected values, that would be a pretty interesting result. For those not familiar with them, Memphis and Louisville are the headquarters for the FedEx and UPS aviation divisions, respectively. They account for a very large share of nighttime jet traffic over the CONUS. They're both high on the list of the busiest cargo airports in the world with almost all of their traffic being at night.

@reirab - No, the sample size is not satisafactory, but in modern times that's likely all you'll ever get; the FAA isn't going to ban all air traffic for a month to run a more controlled longer-term study. Anything that would keep the entire commercial U.S. fleet grounded for longer than a few days is likely going to skew the results in its own way (for instance, a massive eruption or an asteroid impact blanketing the skies in dust or ash)

I cannot find the study from David Travis, only press articles mentioning it. Can you provide a link to the actual study? Also, what Travis said seems to be that the difference between night and day temperatures increased by 1.1°C. It seems he didn't say that the day time temperatures increased by about 1°C. At least this is what NASA understood here. Could you clarify what thesis you are supporting?

@MichaelT: Thanks for the good link. It confirms my reading of the previous information. "data does not suggest that contrails offset or enhance global warming. Other research has suggested contrails have a global-warming effect. But Travis' data addresses variability between day and night temperatures". This is anyway an interesting finding, albeit the 3-day period of the study may not be sufficient.

@VolkerSiegel: The study did not only measure temperature of those three days (nation-wide) with those of the days before (nation-wide). That would indeed be pretty useless. It also measured the relative differences in areas where there's usually a lot of air traffic (just not in those three days), and those where there isn't (all year long). But saying "I don't even need to read it to know it's nonsense" is, of course, your choice to make.

Penn State researchers, led by Andrew M. Carleton, professor of geography, found that jet contrails can affect ground temperatures. (Source)

Bernhardt and Carleton looked at temperature observations made at
  weather station sites in two areas of the U.S., one in the South in
  January and the other in the Midwest in April. They paired daily
  temperature data at each contrail site with a non-contrail site that
  broadly matched in land use-land cover, soil moisture and air mass
  conditions. The contrail data, derived from satellite imagery, were of
  persisting contrail outbreaks. The researchers reported their results
  in a recent issue of the International Journal of Climatology.
They found that contrails depress the difference between daytime and
  nighttime temperatures, typically decreasing the maximum temperature
  and raising the minimum temperature. In this respect, the contrail
  clouds mimic the effect of ordinary clouds.
The researchers report that the "diurnal temperature range was
  statistically significantly reduced at outbreak stations versus
  non-outbreak stations." In the South, this amounted to about a 6
  degree Fahrenheit reduction in daily temperature range, while in the
  Midwest, there was about a 5 degree Fahrenheit reduction. Temperatures
  the days before and after the outbreaks did not show this effect,
  indicating that the lower temperatures were due to the contrail
  outbreaks.
"Weather forecasting of daytime highs and lows do not include
  contrails," said Carleton. "If they were included in areas of contrail
  outbreaks, they would improve the temperature forecasts."

The National Science Foundation supported this work.
https://www.youtube.com/watch?v=7O8FGUtLNfU&feature=youtu.be

"the contrail clouds mimic the effect of ordinary clouds" – They are clouds, aren't they?

yup, but they have a funny shape.

@JörgWMittag: Haha, nice comment.

@JörgWMittag They are clouds but not "ordinary" clouds.

@DavidRicherby: (was looking over my older comments when I saw your reply, sorry for the necropost...) My impression was he didn't mean that seriously, rather it was a joke about chemtrails vs. contrails. Looking at it now though I'm not sure if I was right or wrong.

Yes, contrails affect the weather. The vapour particles are nucleii for condensation of more water. In WW2 the huge bomber fleets leaving Britain caused rain.
The effect of clouds on global warming is still not understood. They reflect the Sun's rays, but they also insulate the ground.

I was thinking of the same thing. Here's an article on the topic, with reference to WW2: http://www.scientificamerican.com/article/contrails-aviation-affects-climate/

They can very slightly affect surface temperatures, just as a similar amount of normal cloud cover would. However, this effect is slight and is not weather modification in the sense that conspiracy theorists claim regarding supposed 'chemtrails.' And, certainly, there is nothing clandestine about contrails. No one attempts to hide them (or, if they are, they're doing a very bad job of it, since it's public knowledge and they're quite easy to see.)
So, to answer your questions, yes, they can have very slight affects on the weather (specifically, surface temperature, usually near major airports or air traffic routes,) but, no they have nothing whatsoever to do with any sort of government conspiracy.

Not in a real sense. yes they do put water vapor in the air. so does drying your clothes in the dryer. or taking a shower. or driving your car. while it makes a visible cloud the amount of water is much less than cars traveling the same distance due to the fuel efficiency of flight vs. driving.

There is one critical point missed in this reasoning: The contrails iced clouds are higher than the water vapor from the dryer. When high the condensation has an effect on the net exchange between Earth and space, known as radiative forcing.

the atmosphere is between the earth and space. the water vapor is in the atmosphere . what exactly am I missing here? Radiative Forcing, sounds more like a football penalty than a scientific term. it would be more like 'holding' or a 'face mask' penalty that 'forcing'. who comes up with this dribble?

Just click the links provided, they will tell you more than I could.


text:
For the same airspeed does the attitude of an airplane change with varying wind directions?
Should one maintain a constant attitude if he wants to hold a constant speed or does the attitude vary depending upon the wind direction?

Yes.  Constant pitch, constant airspeed.  "Pitch plus power is performance, pitch for speed, power for altitude."
Wind affects groundspeed, not airspeed (except for sudden gusts or sheer).
Unless an aircraft has landed or is fastened to the ground, like a tethered balloon or gyrocopter, an aircraft is always moving along with the wind.  Think of an airborne aircraft as being one with the wind.  Wind exists only from the earth's perspective, it has no affect on a flying aircraft.  Except...
Wind does affect heading and groundspeed and becomes important when navigating relative to the earth.  It's especially a factor in landing and taking off!
So, yes.  Constant power, constant attitude will result in constant airspeed, no matter what the wind down on the ground is doing.

If by attitude you mean pitch attitude or angle of attack (which is how I automatically understood this), no it does not change as this is related to the aircraft's airspeed through the air. So an aircraft flying downwind will have the same AOA as the same aircraft at the same airspeed flying upwind.
If you want to fly at the same groundspeed upwind as downwind, then you would have to vary your airspeed, which means varying you pitch attitude or AOA. This might not be possible if say you were in a light aircraft in a 40 knot wind at altitude which cruises at 100 knots.

The lift generated is proportional to the airspeed and the angle of attack (amongst other factors).
If the wind direction changes, the aircraft will maintain a constant airspeed, by increasing or decreasing power.  To maintain level flight, by maintaining the same lift, the angle of attack, for the given weight and altitude, will remain constant therefore there is no attitude change.

why should the aircraft accelerate or decelerate if the change is in direction and not in intensity?

also, even supposing a change in intensity, you want a constant airspeed, so you would throttle up or down to keep the airspeed constant (and thus you would neither accelerate nor decelerate)

@Federico If the direction changes and not the intensity, then the airspeed will change requiring throttle up or down (what I mean by accelerating declerating which was poor choice of words which I'll edit) to maintain constant airspeed since the vector is on the nose.

ah, ok, so we agree on that. but if it changes direction you would have also a change of sidewind component, needing a different yaw angle for compensation.

@Federico Yes, I am assuming constant heading aren't I?  I'll just delete this, it will be easier ;)  Thanks.

If the change is in direction, the airplane will change its yaw (also known as heading) angle, so that the track angle (the direction along which it is travelling) does not change.
For more informations about the differences between heading, track and similar angles, you can read this other question.


text:
How is a prevailing wind/average temperature study conducted to determine the runway direction and length for an airport?
A popular airport near me (KMYF) has two parallel runways, 28/10 R/L and a crosswind runway, 5/23. Almost always, 28R/L is in use. I'm sure they knew that there was generally a sea breeze in the area, and the wind typically will be right down the runway or within 10-20 degrees of centerline.
I'm sure they do prevailing wind study for the area, and my questions are about the study: What are the specifications for the study? Does the study vary in length depending on location? Is there a standard study battery of tests that they use, and do they test things like average temperature during the study to determine runway length (to maximize utility during high density altitude operations)?

it will also depend on available flight-paths around existing infrastructure

Oops!  KMYF has runways 28/10, not 27/10.

@ratchetfreak good point. What gets priority? I mean if they do a prevailing wind study and figure out that it's best to have 28/10, but that traffic pattern puts it into terrain or other hazards, would they switch the runway, or would they look for a new location? I guess my question is, what is all involved in the study? I'd imagine the first thing is looking at existing infrastructure, then figuring out the directions and lengths? I'm interested in a high-level overview of the whole process.

FAA AC 150/5300-13A ("Airport Design") talks a little about this (you want Appendix 2, "Wind Analysis").
Basically you start by figuring out what kind of aircraft you want to operate at the airport, and establishing some basic wind limits (specifically, the maximum crosswind and tailwind components you want to deal with).
Then you call up NOAA's "National Climate Data Center", tell them where you're looking to build your airport, and see if they can have wind data for the location you're looking to build on.

If NOAA doesn't have data you have a couple of other options -- another nearby airport's wind analysis can be used (if the topography doesn't vary that much and the wind analysis is "recent"), or you may have to perform a wind survey which basically means placing a anemometers on the site to record the wind speed and direction for a year.

Armed with collected wind data you tally up the number of observations in given wind speed ranges and mark them down on a "wind rose".
You then place your runway(s) on the wind rose so that they're aligned with the prevailing winds and determine your "wind coverage" - the amount of time the winds at your site are within your crosswind and tailwind limits.
The advisory circular I linked to shows you how to do this by hand, but there's also software to do this which is how most folks do it. The FAA even has a handy web form which will generate the wind rose and runway/crosswind limit bars for you.

Note that this is just the wind part. As you've alluded to there are a number of other considerations, like terrain: If the prevailing winds favor runway 36, but runway 36 points right at a mountain a mile from the field you might have to align the runway 33 or 03 and sacrifice a perfect headwind for not having pilots fly into the mountain.
Also as ratchet mentioned the airspace around your proposed site may influence your runway layout and types of operations. There's a separate FAA office that deals with "Obstruction Evaluation and Airspace analysis" and a process to go through to ensure that your airport is not deemed "Objectionable".
Also as you alluded to in your question there are other considerations like density altitude (local temperature and humidity trends) which will influence design factors like runway length - there's another advisory circular (AC 150/5325-4B) that talks about runway design criteria, including length considerations.

For even more than you ever wanted to know about airport design you can check out the full 150-series of Advisory Circulars, which all deal with airports in some way (there are ACs on design, construction, markings, lighting, ARFF, and even how to qualify for and obtain federal aid).

Not to forget a noise assessment...

@yankeekilo or the all-encompassing environmental impact study.

There are literally an infinite number of studies that may be required prior to constructing an airport (the local municipality may require a study on the effects of runway alignment on the migratory patterns of local slugs). This question is only concerned with wind and temperature though...


text:
Does a primary student, training in a high performance airplane, need an high performance endorsement prior to solo?
A pre-solo private pilot student is training in a Cessna 182 they purchased, do they also need a high performance endorsement, as well as the solo endorsement prior to their first solo flight?

Yes.  In order to solo, a student pilot must act as pilot-in-command of that aircraft, which requires a logbook endorsement for high-performance airplanes in order to do so per §61.31(f).

Additional training required for operating high-performance airplanes.
(1) Except as provided in paragraph (f)(2) of this section, no person
may act as pilot in command of a high-performance airplane (an
airplane with an engine of more than 200 horsepower), unless the
person has -
(i) Received and logged ground and flight training from an authorized
instructor in a high-performance airplane, or in a full flight
simulator or flight training device that is representative of a
high-performance airplane, and has been found proficient in the
operation and systems of the airplane; and
(ii) Received a one-time endorsement in the pilot's logbook from an
authorized instructor who certifies the person is proficient to
operate a high-performance airplane.

would the situation would the same for tailwheel airplane/ endorsement?

Well I don’t know…. What does §61.31 (i) state?

@quietflyer Same situation: 14 CFR 61.31(i) states Except as provided in paragraph (i)(2) of this section, no person may act as pilot in command of a tailwheel airplane unless that person has received and logged flight training from an authorized instructor in a tailwheel airplane and received an endorsement in the person's logbook from an authorized instructor who found the person proficient in the operation of a tailwheel airplane. A non-certificated pilot acts as pilot in command while flying solo, so they would need the tailwheel endorsement.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
Did the NSDAP help to fund Lufthansa?
During WWII, a number of German companies received financial backing and slave labourers from the government at the time. Companies such as Hugo Boss, Volkswagen and Lufthansa.
A thread on Airliners.net from 1999 mentions a documentary showing Lufthansa planes flying with the Swastika on their tail. I found this image online:

What is the story behind Lufthansa's involvement with the National Socialist Party? Did they indeed receive large monetary backing from the then-government?

Nazis just changed the national flag. Any German airline would have no choice in the matter. They may have received money from the government being run the Nazi party, sure, but that doesn't mean they would stop flying when a new party took over. They just change the flag back.

Look up the idea of a national airline, a flag carrier, and drop your attitude that for whatever reason they should have been banned from operating because they had been such.
Aeroflot carried the flag of the USSR proudly, the USSR killed more people than Nazi Germany ever did. Totally irrelevant. They're their country's flag carrier, so they show the flag.
Anyway, this has nothing to do with anything military. Lufthansa was state owned at the time (and indeed into the 1990s), so of course they flew the then current flag of the state, which for a decade happened to include the Swastika.
Also, the post-WW2 Lufthansa is a different company from the pre-WW2 Lufthansa. It was incorporated in 1953 and purchased the name Lufthansa on the open market as a trade name, an old and proud name in European aviation, a name associated with quality and reliability.
They still bear the national flag of Germany to this date, albeit a smaller one. But at the time it was very common for airlines to show the national flag very large and proud on the tail fin or fuselage.

Thanks a lot for your answer! This makes a lot of sense, and answers the question of the flag being flown; but it doesn't touch on any financial support they received. You mentioned the "new" Lufthansa was formed in '53. Was the "old" Lufthansa forced to close down?

@DannyBeckett after WW2 Germany for several years did not exist. Lufthansa was indeed shut down as a result (as were all agencies of the German government). Financial support? They were a government agency, effectively. Thus their finances were part of the government finances.

Mmmm, this response is perhaps just a tad inflammatory.  I will admit, I didn't see the orignial wording on the question as Danny has been kind enough to work with us to make it less about "Nazis" and more about how a wartime government subsidized an airline, which is a legitimate question.  I downvoted this answer, and until it's less argumentative and more to the point of the actual question, that vote will stay down.

@JayCarr You can see the original version of the question by clicking the "edited 7 hours ago" link, and looking through the revisions. My original post is here.

@DannyBeckett Right, but you edited the heck out of the question and have done everything you can to make it as unoffensive as possible.  It's only right that jwenting edit back his rhetoric as well.  Besides, the other answer is a better answer....  If it were me I'd switch.

@JayCarr I never edited anything, I responded days ago to the original text of the question which was highly inflamatory, hinting that Lufthansa should be banned because of Nazi sympathies now simply because they had portrayed the Swastika in the 1930s. My response never was inflamatory, unless you consider pointing out the flaw in that reasoning to be inflamatory.

@jwenting "drop your attitude", "USSR killed more people than Nazi Germany ever did", "an old and proud name in European aviation", "very large and proud on the tail...".  All emotionally charged statements which, in context, are inflammatory.  They add nothing to the answer except a sense of righteous indignation.  Granted, perhaps the original question warranted that.  But, the question has been edited and no longer requires a backlash of any sort.  Again, until this answer is edited to be less emotionally charged, I stand by my down vote.

NSDAP was the elected ruling party in Germany from 1933 to 1945 (only the first part of it elected though); thus Lufthansa was given government money, not "Nazi"-money. During this time, they made the Swastika the flag of Germany. American Airlines sport the American flag all over their tail, and I'm sure they (at some point) have received government money as well. If you replace every occurrence of "Nazis" in your question (including title) with "government", you'll notice it becomes slightly less provocative, and the parallels to other airlines and other countries become readily apparent.
When the war started, I would assume the military requisitioned all the assets they needed including airplanes and services from Lufthansa. I'm not much of an historian though, so I can't give you any details, but I'm sure they're out there to find. I realize this doesn't really answer your question (Lufthansa's involvement with the German government), the point I'm trying to make is that it's very hard to answer, as with any airline and any government.


text:
How do pilots decide what their cruising altitude will be?
When filing a flight-plan for a long-haul high-altitude IFR flight, how do pilots select a specific cruising altitude? Typically in the range of 25,000 - 35,000 ft.

Related: http://aviation.stackexchange.com/questions/991/how-do-i-determine-the-best-altitude-to-fly-for-fuel-economy

Very closely related, if not a duplicate: http://aviation.stackexchange.com/a/1860/69

As has been stated, ATC will assign you an altitude, taking into account your requested altitude, traffic conditions, and of course the FAR.
Peter's formulas will apply from a design standpoint. The aircraft designer will use something like that to calculate typical values, which will be included in the aircraft's operating handbook.
The table below is listed by pressure altitude and gross weight. At each combination there will be performance values such as:

Average N1 (for a turbine)
Max TAT for thrust rating
IAS Knots
Mach number
ISA fuel flow LB/HR/ENG
ISA TAS Knots

The flight distance will determine the approximate gross weight, which corresponds to an altitude and cruise speed that provide the best efficiency. Here is an example from a virtual 737 handbook, with the values I listed. The optimum performance is in blue. Higher gross weights have been removed for clarity.

Airlines may have their own performance tables, choosing a custom balance of efficiency and speed. Dispatchers for the airline will take the flight information and decide on a cruising altitude for filing the flight plan, which the pilot will then receive. As you can see, as the aircraft burns fuel and gets lighter, it will be more efficient at higher altitudes. This is the step climb that Peter mentioned, where the aircraft will climb to higher altitudes as the flight progresses when they are cleared by ATC.
Modern planning systems can take into account weather factors such as winds aloft and turbulence to pick the most efficient route.

Good points. I'll only add that for airline ops (such as the 737 in your A) a dispatcher will do all of this work, not the pilot. I read the cruise altitude off of the dispatch release to see what my dispatcher filed and planned us at.

Note that the winds aloft often override the (modest) effects of performance changes in altitude, since they typically increase in velocity as you increase altitude. Thus, with tailwinds, it often pays to operate as high as the airplane can (given constraints of climb capability, pressurization, service ceiling), and with headwinds at the lower end of the range of reasonable altitudes. It always seems to work out for me that in headwinds they exactly cancel, and the performance gain of flying higher is perfectly wiped out by the increase in headwind component ;)

I guess you know the FAR rules for picking discrete values.
When it comes to performance, with a jet you want to fly as high as possible. The only reason not to do so is total distance; on a short hop there is not enough flying time to climb all the way up.
The higher you fly (in the troposphere) the colder the air is, which makes the thermodynamic cycle of all air-breathing engines more efficient. Also, the air gets thinner, so your friction drag is reduced at the same TAS. The specifics depend on the thrust over airspeed, and generally you want to fly at a lift coefficient between $\sqrt{\frac13\cdot c_{D0}\cdot\pi\cdot AR\cdot\epsilon}$ and $\sqrt{c_{D0}\cdot\pi\cdot AR\cdot\epsilon}$, where $c_{D0}$ is the zero-lift drag and $AR$ the wing's aspect ratio.
The higher value is perfect for piston engines and props, and the lower one for turbojets (think fighter aircraft engine). With a turbofan you will be between both values. To yield more precise results, both formulas will be longer, but these are the major factors for optimum lift coefficient.
Since there is an optimum cl for maximum range, you might want to climb continuously during the flight to compensate for the lower mass (due to fuel burn) with lower air density, so in reality you adjust altitude in steps, in accordance with traffic control.

Cruise altitude may be assigned by ATC for separation (IFR especially, or in other controlled airspace -- since, in the US, these altitudes would be Class A and controlled airspace).  For example, when you request a DUAT(S) briefing, it will usually give suggested ATC routing and also altitude assignments (along with what's most frequently used).
The altitude you request somewhat depends on aircraft performance, I would imagine, but obviously any ATC clearance must be adhered to unless amended.  Hopefully one of the commercial pilots can fill in more of the blanks, though.
See also: Choosing an Appropriate Altitude